The present invention relates to an iron blue pigment, the process for its preparation and use.
Iron blue pigment (DIN 55 906) is known under the designations Berlin blue, Paris blue, Prussian blue, Milori blue, steel blue ferrocyanic blue and bronz blue. In the English-language, iron blue pigment is also designated as "iron blue" (Ullmann's Encyclopedia of Technical Chemistry, 4th ed., Vol. 17, pages 623 ff.)
Iron blue pigment is valued in the printing and lacquer ink industries and for the coloring of fungicides, because of its high color intensity and its low price as compared with organic pigments.
Iron blue pigment is an alkali ferriferrocyanide having the general formula Me(I)Fe(III)Fe(II)(CN).sub.6.H.sub.2 O, where Me(I) is a monovalent alkali metal ion.
The preparation of iron blue pigments is carried out by precipitation from hexacyanoferrate(II) solutions and iron(II) salt solutions via the so-called "white dough" or "Berlin white", which has the general formula Me(I).sub.2 Fe(II)Fe(II)(CN).sub.6.H.sub.2 O, which is optionally oxidized while adding alkali metal salts after a heat treatment (aging).
After a heat treatment (aging) while optionally adding alkali metal salts the precipitation is oxidized.
Only iron blue pigment compounds that contain in their molecules chiefly potassium and/or ammonium ions as the alkali metal have a character suitable for coloring technology and thus have practical significance for the pigment processing industry.
An iron blue pigment is known from German OLS No. 3,038,285, which is characterized by a potassium content of 7.0-9.0 weight %, preferably 7.6-9.0 weight %, and a sodium content of 0.5-2.0 weight %, preferably 0.5 to 1.5 weight %.
The known iron blue pigment is identical to Vossen blue 705/80. Intaglio printing inks prepared therewith are up to 15% weaker in color and contain no red fraction.
It is known that the nature and quantity of alkali metal present in the Berlin blue lattice are not only the cause of the color strength and depth, but that they also affect the hue. What is meant in particular is the undertone which becomes visible in transparent layers. The color of the alkali metals present in Berlin blue obviously corresponds to their spectral lines in the coloring of flames. The alkali metals used in the preparation of this blue pigment on a large scale are sodium potassium and ammonium. Sodium has yellow spectral lines, potassium violet, and ammonium strong violet lines.
Known red-tinged blue pigments are manufactured with the general designation "toning blue". The red hue is achieved by an adsorption of NH.sub.3 on the pigment surface. NH.sub.3 is formed from the ammonium salt present in the suspension during the white-dough oxidation in the alkaline region.
The oxidation is carried out with sodium dichromate. Chromium remains as Cr(OH).sub.3, along with the Fe(OH).sub.2 [Fe(OH).sub.3 ], which is likewise precipitated, as a disturbing compound in the pigment. The great disadvantage of these types of toning blue lies in the fact that NH.sub.3 is dissolved from the pigment binder. In addition, NH.sub.3 is displaced by the effects of heat. In this way, the red portion of the pigment disappears immediately. Because of its instability, this coloring agent is no longer manufactured.